Introduction

Sometimes it is required that you export calculated data from one environment to another. The same thing occurred when you are working with C/C++ and also MATLAB environment. In this case, some tools must already exist to act as a bridge for both environments. MATLAB C MAT-File APIs are a group of functions for reading/writing a MAT compatible file from C/C++ environments that can be read by MATLAB. This means that if you save your variables data (result of some calculation) from MATLAB, you can read them from a C program and continue your calculation and if you compute something in your program and want to save them in a file that MATLAB can read them without any problem, you can use these APIs.

C MAT-File APIs

MATLAB C MAT-File APIs are those functions for opening a file and saving data in a compatible format for using them in MATLAB. The first step to use these functions is including mat.h header file. Notice that if you want to use MATLAB built-in functions, you must include matlab.h header file too. The second step is adding MATLAB libraries to your project. The required library is libmat.lib. If you use another MATLAB APIs, add their libraries too. For example, if you use mlfPrintMatrix function to print matrix elements, you must add libmx.lib, libmatlb.lib and libmmfile.lib.

Below are some of basic C MAT-File APIs with their description and syntax:

MATFile *matOpen(constchar *filename, constchar *mode);

Arguments:filename is name of file to open, mfp is a pointer to MAT-file information and mode is file opening mode.

Legal values for mode listed in Table 1:

r

Opens file for reading only; determines the current version of the MAT-file by inspecting the files and preserves the current version.

u

Opens file for update, both reading and writing, but does not create the file if the file does not exist (equivalent to the r+ mode of fopen); determines the current version of the MAT-file by inspecting the files and preserves the current version.

w

Opens file for writing only; deletes previous contents, if any.

w4

Creates a MATLAB 4 compatible MAT-file.

Description: This function allows you to open MAT-files for reading and writing. matOpen opens the named file and returns a file handle, or NULL if the open fails. You must close the file by using matClose function.

char **matGetDir(MATFile *mfp, int *num);

Arguments:mfp is a pointer to MAT-file information. num is address of the variable to contain the number of mxArray variables in the MAT-file.

Description: This function allows you to get a list of the names of the mxArray variables contained within a MAT-file. matGetDir returns a pointer to an internal array containing pointers to the NULL-terminated names of the mxArrays in the MAT-file pointed to by mfp. The length of the internal array (number of mxArrays in the MAT-file) is placed into num. The internal array is allocated using a single mxCalloc and must be freed using mxFree when you are finished with it. matGetDir returns NULL and sets num to a negative number if it fails. If num is zero, mfp contains no arrays. MATLAB variable names can be up to length mxMAXNAM, where mxMAXNAM is defined in the file matrix.h.

Arguments:mfp is a pointer to MAT-file information and name is name of mxArray to get from MAT-file.

Description:matGetVariable allows you to copy an mxArray out of a MAT-file.matGetVariable reads the named mxArray from the MAT-file pointed to by mfp and returns a pointer to a newly allocated mxArray structure, or NULL if the attempt fails.matGetNextVariable allows you to step sequentially through a MAT-file and read all the mxArrays in a single pass. The function reads the next mxArray from the MAT-file pointed to by mfp and returns a pointer to a newly allocated mxArray structure. MATLAB returns the name of the mxArray in name.

Use matGetNextVariable immediately after opening the MAT-file with matOpen and not in conjunction with other MAT-file functions. Otherwise, the concept of the next mxArray is undefined.

matGetNextVariable returns NULL when the end-of-file is reached or if there is an error condition.

In both functions, be careful in your code to free the mxArray created by this routine when you are finished with it.

int matPutVariable(MATFile *mfp, constchar *name, const mxArray *mp);

Arguments:mfp is a pointer to MAT-file information. name is name of mxArray to put into MAT-file. mp is an mxArray pointer.

Description: This function allows you to put an mxArray into a MAT-file. matPutVariable writes mxArraymp to the MAT-file mfp. If the mxArray does not exist in the MAT-file, it is appended to the end. If an mxArray with the same name already exists in the file, the existing mxArray is replaced with the new mxArray by rewriting the file. The size of the new mxArray can be different than the existing mxArray. matPutVariable returns 0 if successful and nonzero if an error occurs.

Importing Variables Data From MATLAB

For saving variables in MATLAB, just use save command. for example:

save myFile

By entering the above command, MATLAB will save all of variables in its workspace in a file named myFile.mat. For saving only some variable, use save command like:

save myFile X Y Z

with above command, MATLAB saves only X, Y and Z variables. Now we want to use C MAT-File APIs to read myFile.mat and extract all of saved variables. Below is our C code to doing this:

Exporting Variables Data To MATLAB

It's time to do reverse task. In other hand we want to calculate something and save data to a file that MATLAB can read it. To import data from a MAT-File to MATLAB environment, one must use load command:

load myFile

Load command, load workspace variables from a file located on your disk. Following source code, define some mxArray varialbe and then save them in a file. This file can be called from MATLAB environment.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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About the Author

I was born in Shiraz, a very beautiful famous city in Iran. I started programming when I was 12 years old with GWBASIC. Since now, I worked with various programming languages from Basic, Foxpro, C/C++, Visual Basic, Pascal to MATLAB and now Visual C++.
I graduated from Iran University of Science & Technology in Communication Eng., and now work as a system programmer for a telecommunication industry.
I wrote several programs and drivers for Synthesizers, Power Amplifiers, GPIB, GPS devices, Radio cards, Data Acqusition cards and so many related devices.
I'm author of several books like Learning C (primary and advanced), Learning Visual Basic, API application for VB, Teach Yourself Object Oriented Programming (OOP) and etc.
I'm winner of January, May, August 2003 and April 2005 best article of month competetion, my articles are:

Comments and Discussions

i am working on linux platform , i am doing a project its main task is developing a tool in c++ that can capture the real-time data network streams and find the bitrate of that stream , after that i have to calculate the mean and average for that streams in different locations.i have completed up to finding the bitrate and stored that bitrate information in a file.can anybody give the information how to read this real-time data in to MAT-LAB to find the mean and average .????

I have a problem in reading a mat-file by use of the C++ language (in
fact to make a S-funtion to use by simulink). For this I have a file
in which there is a variable called lookuptables. In there there is
are 4 structures arranged in the vertical direction. in each
structure there are 3 variables: Index,values and size. the size I
can read in but for the index and values I cannot. When I look in
matlab at these variables they look like for example: [10 45 19 56
87].

Looking at the API, it doesn't seem possible to just write one array on the fly. Instead, you have to collect your data, then write it out.
Unfortunately I just tried a Simulink file writer block which generates a .mat file and bumped into a bug directly; from Simulink it works ok but a compiled C version (which just writes a header and then appends time/value double pairs at every timestep) writes a bogus file (loading it will give silly numbers).

Any idea of how I could efficiently implement realtime writing of a value? (buffering all the data seems like implementing a file all over again ).

Thanks for any tips, it just seems a bit non-realtime, the Matlab standard API.
Ruud van Gaal

To follow up on myself, I now write an older Matlab version, which enables you to generate values on the fly, instead of having all values ready to write.

I'll include the useful functions, writing a header and appending values. Note that you will have to rewrite the header upon closing time! This will enable you to set the right array size.
Hope this helps anyone.

Selam;
Thanks for your articles please continue. I'm a new member. I work about MATLAB C compiler. I've written the function below and compile it with LCC. But after running the created EXE, result is not good. File contains no data(I understand from its size) and MATLAB cannot import resulting MAT file. Gives the error
"Error using->getfield"
"Error using->subsref"
"Invalid file name component"
I cant understand what is wrong ? Could you please help me? I use MATLAB 6.1.0.450 (R12.1).
Thanks
Regards
Fuction is below;

i found this article very helpful..but i have a few problems implementing it.first #include "matlab.h" is resulting in an error mssg. "unable to open include file "matlab.h"" though it's there in the same dir.
the purpose of my program is to extract an output from a matlab program and use it in C to (hopefully, eventually!) move a robo,so i need a realtime array of variables from matlab..is it possible?
please help.
i'm not a software expert (or anything!), so pleae reply in layman terms.
thankyou.

Hi
Thank you for this article and I think it is what I was looking for.

I need to export float data from matlab to my C program.
So I think the program you gave does that.
I installed mat.h and nothing else.
Do I need libmat.lib ? where can I download this?
I have a error : Undefined linking perhaps it is because I didn't use libmat.lib ?? It doesn't work.

Thank you very much for your excellent article. I am not a Matlab user (Matlab is not installed on my computer) and just want to read a MAT-file. So my question is: what files are needed and where do I find them? I think that I would just need mat.h, libmat.lib and some dll that is associated with the .lib-file. I have the mat.h file (I installed the MCRInstaller) but it seems it didn't install the other two files.

First of all, thank you for the reply.
It was only a test. I created an m-file that calls linspace and built a COM object using "comtool". So, with an integer input, it created a matlab vector. Using your article "Building COM Components Using MATLAB - Part I" and a m-file that calls plot, I plotted this vector. I don't know (hope yet) how to create a vector in C# and than use the plot function. Please, do you know how to do that too?

Unfortunately I have no experiences developing MATLAB applications in C#. But I think you can follow the rules that I mentioned in two artilces of "Building COM Components Using MATLAB" to create vectors in C#.

i did much search but did found any clue about quick sorting algorithm which is implimented in Mat Lab. If u have any tutorial or sample source code about this plz send me i have really need about this.